crd struct management and versioning doc

[clux]

dealing with incompatible crd schemas / serialisation issues / versioning is a bit complicated atm and our solutions is hidden in obscure areas here and there in the codebase, a joining doc might be helpful here.

things worth covering (or at the very least linking to):

• multi versioning ala examples/crd_derive_multi and merge_crds
• dealing with incompatibilities using the new DeserializeGuard from Add error boundary wrapper type kube#1556 with an actual implementation usage in Allow other NifiClusters to reconcile even if one is invalid stackabletech/nifi-operator#662
• conversion webhooks via Support ConversionReview for webhooks kube#865 - find users?

anything else?
not planning on doing this immediately - should at least release DeserializeGuard etc first.

EDIT: 0.94 has more on this now. I put some basics in the object doc;

• c54c9db
• 26a6cab